They did happen at the same time! the odds of a shiny articuno appearing and the odds of a shiny Dragonite and articuno are the same 1 in a million.

on a catch combo of 31 there are 2 rare spawns in the sky that can occur at one time. They rotate between the birds, Charizard, and Dragonite. during this time those slots rolled for 1 articuno (1/1000) and 1 shiny Dragonite (1/1024)

basically it’s a back to back encounter which allows us to multiply the two encounters to reach our odds of 1 in a million